In wireless communication terminals, such as mobile phones and USB modems, modulating the supply voltage of the power amplifier (PA) in such a way as to follow the envelope of the transmitted signal allows the PA to be operated at a higher efficiency. This technique is known as envelope tracking (ET). The circuitry/component that generates the PA supply voltage in this type of system is called an ET modulator.
Turning to FIGS. 1A and 1B, there are shown diagrams illustrating output voltage (Vout) during power amplifier operation when supply voltage VCC is static (FIG. 1A) and when VCC tracks the envelope of Vout (FIG. 1B). In general, when the supply voltage VCC of the power amplifier is lowered, power consumption decreases, therefore increasing efficiency. However, its non-linearity also increases, which degrades the output signal.
When VCC is static, maximum efficiency for nominally linear operation is achieved when VCC is equal to the peak value of the output voltage Vout, as shown in FIG. 1A. When VCC is modulated such that it follows the envelope of Vout, as shown in FIG. 1B, then nominally linear operation is maintained. However, the average value of VCC is lower than if it were static, therefore the average power consumption of the power amplifier is lower and efficiency increases. Thus, there are benefits to utilizing this envelope tracking (ET) technique in power amplifier operation.
Turning now to FIG. 2, there is shown a block diagram of a portion of the circuit/components of a prior art transmitter system 100 within a wireless communications terminal/device. Although other circuitry/components may be included in the transmitter system 100, only those portions necessary and relevant for an understanding of the present disclosure are shown therein. Input signals “ID” and “QD” are the digital in-phase and quadrature components of a baseband signal. Digital-to-analog converters (DACs) 110 convert these signals to corresponding analog components “IA” and “QA”. A transceiver 120 (which may include a transmitter) converts IA and QA to a radio-frequency (RF) signal “X”. RF signal X is input to a power amplifier (PA) 130 for amplification to generate an RF signal Y at a power level required for transmission from an antenna (not shown) of the transmitter system 100.
Signals ID and QD are also input to an envelope generator 140 that generates an envelope waveform “E” of the transmitted signal. An envelope tracking (ET) modulator 150 receives the waveform E and generates a power amplifier (PA) supply voltage VCC (from the primary supply Vsup of the device) using a switching regulator (also known as a switched-mode power supply or switcher). For an “ideal” ET modulator, VCC is identical to the waveform E. For a non-ideal ET modulator, the waveform E can be adjusted (predistorted) to compensate for frequency response and non-linearity of the ET modulator 150 so that VCC more closely corresponds to the waveform E.
Now turning to FIG. 3, there is shown a typical prior art integrated circuit (IC) implementation of a switching regulator 151 in the ET modulator 150, along with a typical prior art filter 155. The switching regulator includes a high side/low side switch 154 which includes transistors M1 and M2 (forming a high-side switch) and transistors M3 and M4 (forming a low-side switch). Typically, a thin oxide is chosen for the switching transistors M1 and M4 in order to allow a high switching frequency at low power consumption. For the cascode transistors M2 and M3, whose gate voltages are held at constant values VcascH, VcascL, a thick oxide is chosen in order to allow operation at values of Vsup that exceed the maximum voltage rating for thin-oxide transistors.
A pulse width modulation (PWM) generator 152 drives the gates of the switching transistors of the switch 154 with non-overlapping (break-before-make) high-side and low-side waveforms PWMH and PWML, whose pulse width as a function of time corresponds to the envelope waveform E. At the output of the switching stage, a low-pass inductor-capacitor (LC) filter 155 formed by L1, C1, L2, C2 and the resistance Rload (i.e., the load resistance presented to the filter by the power amplifier) removes the high-frequency components of output voltage Vsw output from the switching regulator 152 in order to generate the required power amplifier supply voltage waveform VCC.
As will be appreciated, the frequency response of the LC filter depends on the value of Rload, as illustrated in FIG. 4. For a given Rload, the inductor and capacitor values can be chosen such that the frequency response is optimal (i.e., it has sufficient flatness within the bandwidth of the envelope signal and provides sufficient attenuation of the higher-frequency components of Vsw that are generated by the switching process). In practice, however, Rload is a decreasing function of E and VCC and typically varies over a 10:1 range. Because of this, if the filter is optimized for a load near the middle of this range, the frequency response will exhibit excessive peaking during negative excursions of the envelope and excessive droop during positive excursions of the envelope.
One additional consequence resulting from a variation of Rload with the power amplifier supply voltage VCC is the non-linearity in the overall transfer characteristic from E to VCC because the gain (or attenuation) is a function of Rload. Both this non-linearity and the varying frequency response described in the preceding paragraph cause degradation of the transmitted signal.
For a given power amplifier (PA) impedance characteristic, the impact on the frequency response and non-linearity of the ET modulator 150 can be compensated by adjusting (predistorting) the input waveform E of the ET modulator 150. However, because the power amplifier impedance characteristic varies from part to part due to manufacturing characteristic(s) and tolerance(s), it is desirable to measure the power amplifier impedance characteristic of each device/terminal as part of a factory calibration that determines the necessary compensation. In such a process, to perform this measurement, the load current of the ET modulator 150 needs to be sensed.
In one prior art method, the need to measure the load impedance characteristic is avoided by adding a feedback loop, as shown in FIG. 5, that includes a loop filter 160 and summer/adder 161). If the loop gain and loop bandwidth are sufficiently high, the feedback will suppress deviations of VCC from signal E resulting in impairments in the transmitted signal that are acceptably low. The purpose of the loop filter 160 is to provide sufficient loop gain with a frequency response that ensures stability over all variations in Rload. This is relatively straightforward for a switching regulator with a static output, since the bandwidth can be made arbitrarily low. However, for the ET modulator 150, the high bandwidth requirement (e.g., up to 20 MHz for a long-term evolution (LTE) cellular standard) presents difficulties in the design of the loop filter to achieve sufficient loop gain and bandwidth for the feedback to be effective while maintaining stability over the full range of variation in the load impedance (typically around 10:1, e.g. 4Ω to 40Ω).
Due to the trade-offs between loop gain, bandwidth and stability, most other prior art solutions need an added error amplifier 162 (as shown in FIG. 5) to achieve acceptable transmitter performance. Both the loop filter 160 (which has to be an active filter) and the error amplifier 162 draw power from the primary supply voltage, which reduces the overall efficiency of the system 100 and greatly diminishes the advantages of using envelope tracking.
One challenge in designing a solution that uses envelope tracking is the fact that the load impedance which the power amplifier (PA) 130 presents to the ET modulator 150 varies with the power amplifier supply voltage and with the envelope of the transmitted signal. This causes both variation in the frequency response of the ET modulator 150 and non-linearity in its transfer characteristic, both of which result in degradation of the transmitted signal.
Accordingly, there is a need for a transmitter system that enables the load current of an ET modulator to be sensed, and hence the load impedance of a power amplifier (PA) to be measured. This will enable suppression of the impedance characteristic by calibration and predistortion instead of by feedback.